The mig process is used to weld most metal types. In cases where weld quality is not critical the Mig process is fast, cost efficient, and produces results that are more than acceptable for most manufacturing and fabrication needs.
MIG welding was developed in 1940’s, and is considered semi-automated. Accounting for the skill of the welder, the continuous filling of the joint being welded through the MIG welding machine, allows for longer weld beads and increased productivity.
Three things are required for the MIG weld process, electricity to produce heat, an electrode to fill the joint, and shielding gas to protect the weld from the air. The wire runs through the liner, which also has gas feeding through the same cable to the point of arc, which protects the weld from the air. MIG welding is done using a very small electrode that is fed continuously, while the operator controls the amount of weld.
TIG welding is mostly used for critical weld joints, welding metals other than common steel, and where precise, small welds are needed.
It is a manual welding process that requires the welder to use two hands to weld. One of the major differences of TIG welding process is the way the arc is created and how the filler metal is added. During the TIG Welding process one hand is used to hold the TIG torch that produces the arc and the other hand is used to add the filler metal to the weld joint. Due to two hands and the coordination required to weld TIG welding is the most difficult of the welding processes to learn. The TIG welding process is the most versatile when it comes to welding of different metals and produces the higher quality of weld , when executed properly, but the process is slow.
TIG welding requires three things, heat, shielding, and filler metal. The heat is produced by electricity passing through the tungsten electrode by creating an arc to the metal. The shielding comes from a compressed bottle of gas that flows to the weld area to protect it from air. The filler metal is just a wire that is dipped by hand into the arc and melted.
The marriage of the three required elements begins with the welder turning on the gas flow, either by or a valve or on the TIG torch itself. The gas begins to flow and starts protecting the weld area from the air. The torch is held over the weld joint just far enough for the torch not to touch the metal. The welder then presses a foot pedal and the TIG torches tungsten electrode starts an arc. Once the arc is started the two pieces of metal begin to melt by creating a puddle of metal. Once the puddle is established the welder with the other hand starts filling the joint by manually dipping a welding wire into the arc to fill the joint. Ultimately this process creates a single piece of metal.
Reference material : http://www.gowelding.org/